abspath.h: move absolute path functions from cache.h

This is another step towards letting us remove the include of cache.h in
strbuf.c.  It does mean that we also need to add includes of abspath.h
in a number of C files.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/fsmonitor--daemon.c b/builtin/fsmonitor--daemon.c
index 215e381..4efb141 100644
--- a/builtin/fsmonitor--daemon.c
+++ b/builtin/fsmonitor--daemon.c
@@ -1,4 +1,5 @@
 #include "builtin.h"
+#include "abspath.h"
 #include "alloc.h"
 #include "config.h"
 #include "gettext.h"